


A Costly Mistake

by CaptainParisStarr



Series: Genderbent Merlin Trilogy [1]
Category: Merlin (TV)
Genre: Arthur Knows About Merlin's Magic (Merlin), Arthur loses control, Dubious Consent, First Time, Gender-swapped Merlin, M/M, Polyamorous Arthur
Language: English
Status: Completed
Published: 2020-09-01
Updated: 2020-09-01
Packaged: 2021-03-07 03:27:19
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,925
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26240062
Author URL: https://archiveofourown.org/users/CaptainParisStarr/pseuds/CaptainParisStarr
Summary: Merlin's magic goes wrong when he tries to use an aging spell for the first time, and turns himself female instead.Episode tag- Queen Of Hearts
Relationships: Merlin/Arthur Pendragon (Merlin)
Series: Genderbent Merlin Trilogy [1]
Series URL: https://archiveofourown.org/series/1907440
Comments: 3
Kudos: 164





	A Costly Mistake

The aging spell did not go according to plan at all.

"Oh no! No, no, no!" 

"Merlin? Are you okay in there?"

"Gaius! The spell didn't work! Well, it worked but…" He groaned as he looked down at himself.

He took a step, and stopped, his balance gone, and the lack of feeling his cock swing between his legs really cemented home just how badly he had fucked up.

He wasn't old; he was a girl. He no longer had a cock; but he did have a perky pair of breasts pushing out the front of his shirt.

"But what?" Gaius pushed the door open, took a step into the room, saw Merlin, and stopped. "Oh dear."

"Is that all you have to say?" Merlin sighed, then he straightened, and carefully walked to the mirror, regaining his sense of balance with every step. "Wait, no one will recognize me like this, will they?"

"I don't know; you still look like yourself just… softer. You should grow your hair out and change its color." Gaius grabbed first one book, then another.

"Agreed." Merlin groaned, then when he was certain Gaius was not looking, grabbed his new chest.

_ "Merlin! _ I know that the new body of yours is strange, but please refrain from inappropriate acts." Gaius glared briefly, then went back to his reading.

"Yes." Blushing, he dropped his hands. "I guess I'll need a dress?"

"Yes, you will." Gaius went to a cupboard and removed a simple servants dress. "Try this on."

"Why do you have a dress?"

"I have several because sometimes I have to cut clothing away to treat a wound, and when women are involved, I have to preserve their modesty somehow.”

"Ah." Merlin felt like an idiot; he had of course seen him do just that, only never to such an extreme. Ducking back into his room, he changed as best he could. "Now how do I look?"

"Turn around." Gaius refastened the ties correctly. "You look like a serving girl."

"Great, just what I never wanted." Merlin groaned.

"Here, this spell should work." Merlin recited the words on the page, and soon his hair was hanging to his waist in surprisingly noticeable waves; another spell, and it was a dark, fiery red.

"There is still something of you in the eyes, but that might just be because I know it's you."

"Did you make the poultice?" Merlin tied the sides of his hair back, hating the way it now tickled his cheeks.

"Yes, here.” He handed over a small sachet, which Merlin tucked away. “Oh and Merlin, be careful!" Gaius cautioned. "Don't do anything that could get you stuck like that."

"Like wh-" Merlin's eyes went huge as he clued in. "No! No way  _ that _ is happening! I like women!"

“Good. Then you have nothing to be worried about, now do you?” Gaius nodded, as satisfied as he ever would be with Merlin’s disguise.

“Well, wish me luck!” He checked that the poultice was secure, and left the room.

  
  


* * *

  
  


Somehow, against all the odds, he got to Arthur’s chambers, when he suddenly realized a critical flaw in his plan; unlike the robes he would have worn as an old man, he’d had to strip off his male clothes before donning the dress which meant  _ he was completely naked under the dress and had nothing to change into after returning to himself! _

He was just cursing his oversight when Arthur caught up to him. “Who are you?”

Partially turning, Arthur’s eyes were drawn to the poultice. “Are you the one who…” He stopped talking, looking closely into Merlin’s eyes, then suddenly Merlin was on the bed.

“What is this?” Hands groped Merlin’s chest before loosening the bodice enough to pull down to his waist, and roughly caressing the now bare mounds of his new breasts, then the skirt was pulled up, and the hand was between his legs.

“Wait, Sire! Are you this disrespectful with all the serving girls?” He made his voice as feminine as he could.

“Cut the bullshit, Merlin! What sort of trickery is this?”

Whatever Merlin had thought up to reply to any situation left him in a rush, and he dumbly gasped out; “How?” Then he blinked, and pulled back. “I don’t know anyone by that name!”

“I know you better than anyone, except maybe for Gaius. Did you really think this would fool me? What’s your game here, Merlin? Why did you make yourself female? I didn’t even know you could do that with magic.”

“M-magic? I don’t know what you’re talking about!” Merlin’s chest heaved, and Arthur’s eyes were drawn back to the breasts he’d made no effort to cover.

“Also, any real girl would have shrieked and covered themselves with their arms rather than just… let me look…” Arthur suddenly pinked, and turned away. “Why are you wearing nothing but a dress? And what is with this?” He held up the poultice. “You don’t seriously mean for me to believe you planted it… as a girl?”

“No! Gaius and I we… discovered the identity of the magician who planted the true poultice, but we can’t expose them. They are someone in a position of power, and Uther would never believe it of them. So, we decided to make a fake sorcerer, and after I was caught planting the second poultice, I was to break the enchantment, and no one would be the wiser that it was me.”   
  


“So, that’s why you’re a woman? Whose idea was that? Your’s? Gaius’s?”

“It was an aging spell, okay?! Something went wrong; turning me female was never the plan! But once it happened, we decided to roll with it, and now I’m mostly naked as a girl in your bed, and  _ holy shit Arthur are you hard?!” _ His eyes went huge as he took in how tented the Prince’s pants were.

Instead of answering, Arthur sighed, climbed on top of Merlin, and kissed him deeply.

“Wha…?” Merlin blinked up at him stupidly as the kiss broke. “But… Guinevere?”

“It’s… complicated. You know my feelings for her are real, but I… I’ve never been able to just look at one girl. I’m scared to marry her, even if it was allowed, because she would never understand that I want more than one girl in my bed and I…” He took a deep breath before finishing. “I also want boys in my bed.” His voice grew gentle as he looked Merlin in the eye. “I want you in my bed, Merlin. You don’t have to become a girl to catch my eye; you already have it.”

“The  _ last _ thing I was trying to do was  _ catch your eye!” _ Merlin was all but frantic to get away, and yet his body refused to move as Arthur’s hand caressed him once more between his legs, and a strange-yet-familiar warmth began to spread through his body.

“Really? You’re awfully wet for someone who doesn’t want to be in my bed.”

Merlin gasped as he realized just what the warmth was; it was arousal, and had only felt odd because he no longer had a cock to harden. He also could not deny Arthur’s statement, as the Prince licked fluids from his fingers.

“Just… be honest with Gwen. Don’t go behind her back; maybe there is someone she might be okay with?”

“You?”

“Lancelot; I think she’d be with him, if it wasn’t for how she felt about you. But, she could never hurt you by… that’s not Gwen.”

_ “Lancelot?!  _ Merlin, are you certain of that?” Arthur’s eyes shone bright with hope.

“As certain as I can be…” He gasped as Arthur began to untie his pants. “W-what are you doing?”

“I told you Merlin; I want you.” He pulled his pants open, and Merlin let out a strangled sound, half of fear, and half of anticipation.

“It’s big, I know, but I can use it gently. Trust me.” He lay back down on top of Merlin, and the warlock's last chance to escape vanished as his legs were wrapped around a trim waist, and something pushed inside his new opening.

All he could do was cling to Arthur as he felt the Prince move inside him, in ways he had never imagined he would experience. “Are you okay?”

“Yeah, I’m fine. I just… this really wasn’t my plan!” His hands tightened on Arthur’s shoulders as the Prince hit a spot deep inside him that made him scream with sudden pleasure.

“Ah, there we go.” His pace sped, and he angled himself to hit that spot with every second or third thrust.

Merlin’s eyes slid closed as his head fell back; he had never wanted to be fucked by Prince Arthur, but now that he was here, he found he didn’t want to be anywhere else. Pleasure beyond anything he had ever known before sizzled up and down his spine, and that was when a wet mouth closed around one of his nipples.

“Ah! Arthur! Harder! More! Yes.... YES! ARTHUR!” He was barely even aware of speaking, the words pouring from his throat amidst a cacophony of lewd sounds that would have humiliated him under any other circumstance, but at that moment he couldn’t even think, let alone feel humiliation.

Everywhere Arthur touched him, with hands, mouth, or cock, felt hot and tingly, and his nerves sang with every motion Arthur made, amplifying even the lightest caress until nothing else mattered but the boy on top of him, and the sheets below.

Deep in his core, the pooling heat grew, became somehow more, and something began to tighten, turning his breath to sharp pants, and making him cling tighter to Arthur than he had ever imagined possible.

“Merlin, my God but…” Arthur broke off, shaking his head, then when Merlin whimpered. “It’s okay. Let go. Trust me and let go. Let it happen.”

Merlin nodded though he didn’t hear anything Arthur had said, and the tension deep in his core snapped, and he spasmed as wave after wave of pleasure more intense than anything he could have imagined rolled over him.

Heat spread inside him as he went limp, and Arthur pulled out, lying beside him. “Oops.”

“What do you mean, oops?” Merlin frowned slightly, then he felt something drip out of him. “Arthur! You did not…!”

“I tried to pull out, but you were holding me too tight. Merlin, I’m sorry.” He was kissed gently. “Lie here a moment, catch your breath, then we can clean up and plan what to do next.”

“Next?”

“Gwen is still imprisoned, and sentenced to be executed tomorrow. I can’t let that happen!” He sat up, and dropped his head into his hands, sweat still drying on his body from his lovemaking with Merlin. “So if we can’t reveal the true magician, then…” He sat up. “Merlin, you said the plan was originally for an aging spell, right?”

“Yes?” Merlin, slowly sat up, aching in ways he had not known possible.

“Okay, I know what we have to do. After you get cleaned up, hide behind the screen. I’ll call for the guards, and tell them an old man just came in here to plant a poultice and fled, dropping the evidence on the way out.” He dropped the fake sachet beside the servants entrance. “They chase this non-existent old man, giving you a chance to get back to Gaius and return to being a boy.” Arthur returned with a damp cloth, with which he gently wiped Merlin down.

“Oh shit!” Merlin fell back.

“What is it now?”

“Gaius warned me not to do anything that might get me stuck like this, especially… what we just did.” He covered his burning face with his hands.

Arthur’s eyes went huge. “You mean that, like this you could, potentially, get pregnant?!”   


“I don’t know, but maybe? We can’t rule it out.” Merlin peeked out at Arthur from between his fingers.   


“Why didn’t you tell me this earlier?!” Arthur was a mix of livid, and anxious.

“You didn’t give me a chance to say no let alone…”

“No? You mean you didn’t want to?” Arthur turned as pale as milk.

“I… Don’t know anymore. I didn’t at first but… it wasn’t bad. Actually, it was the most amazing thing I’ve ever experienced in my life.” Merlin slipped into Arthur’s arms, pressing his face into the Prince’s chest. “I’ve never thought of men that way; all my life I have only liked women, but… now I don’t know anything anymore. Maybe I do like you, the way you like me, or maybe it’s this body. I just don’t know. No, I do know one thing; if I had really, truly wanted you to stop, I could have said so.” This time, it was Merlin who initiated the kiss.

“I’m sorry, Merlin. I never act this way. I don’t know what came over me.” He rubbed the warlock’s back gently.

“You were overwhelming; I couldn’t think, let alone move or speak. And to be quite honest, I completely forgot about Gaius’s warning until now.” One last soft, tender kiss, and Arthur pulled back, standing up. “We should get the plan in motion.”

“Right!” Merlin stood on shaky legs and pulled up the bodice of his dress.

“Do you need help?”

“Yes, if you don’t mind.” Merlin gasped as the feeling of Arthur’s fingers against his bare skin as the ties were done up sent liquid heat squirting down his thighs.

“Down boy; we can’t risk doing it again.” Arthur’s voice was heavy with lust. “Maybe, once you’re male again, if you still want to…” Arthur trailed off. “But only if you want to; I won’t force you a second time. You have my word as a Knight of Camelot. Merlin, I love you.”

“What?” Even though the words were seared into his brain, he had a hard time believing that he had heard the Prince right.

“Behind the screen.” Arthur gestured for Merlin to move, the mood gone.

“Right.” He hid for what felt like days, as the guards rushed into the room, found the second poultice, and left again, running after a man who did not exist.

“Now.”

He stood and left, not daring to look at Arthur as he ran for his life all the way back to his bedroom, only to discover that he could not break the enchantment.

“Gaius!” He was almost in tears.

“What is it?”

“The spell, I can’t break it! Arthur overpowered me and we…!” He couldn’t say it.

“Did he know it was you?”

“Yes, yes he did. He… somehow he knew about my magic. He thought this was deliberate to seduce him, or something.”

“Hang on; I do know of an antidote. Stay in your room while I brew it. Wait, did the plan work?”

“The guards are chasing phantoms, so yes, it worked. Arthur wants Gwen free as badly as we do.”

“And Morgana?”

“He doesn’t know it was her; all I said was that the magician was in a position of power and could not be exposed.”

“Alright.” He allowed Merlin to return to his room where he collapsed onto his bed, and almost instantly fell into a deep sleep.

* * *

  
  


“Wake up, it’s ready.” He blinked groggily as his shoulder was shaken.

“Gaius?”

Soft lips pressed against his own, and with a gasp he woke up fully. “Arthur!”

“Gaius thought I should be the one to give you this.” He held out a vial. “Something about me needing to take responsibility for my actions if it fails?”

“I told him.”

  
  
“Right. Of course you did.”

“When my magic…” He broke off, covering his mouth as his eyes went huge.

“Merlin, I’ve known for months; I’m not an idiot. It’s a little late to pretend you are not a sorcerer now, don’t you think. You couldn’t break the spell on your own?”

“No, I couldn’t and so I told Gaius. I have never been so scared in my life. What if the antidote doesn’t work?”

“Then… we will figure something out. I promise.”

Merlin settled back as he drank the vial in one gulp, needing to move quickly before the fear could paralyze him, and almost instantly his skin began to crawl, and his body reshape itself.

Around the corners of his vision, he saw his hair darken, and his breasts flattened back out to nothing, as his manhood dropped heavily back between his legs, but one thing did not change.

“Damn! I need a haircut!” He ran his fingers through hair that still reached his waist, even though every other enchantment had been broken.

“Have dinner with me, the day after tomorrow. Tell me then if you still want me.” 

“Alright.” In his relief, he grabbed Arthur for a hug, and for once the Prince allowed it.

“Merlin?” Gaius peeked into the room, and shook his head. “I’ll go get the scissors.”

“Day after tomorrow. Don’t miss it.” Arthur kissed his cheek, and was gone.

“Date night?”

“What? No! Why would you think that?” Merlin sat down with his back to the physician.

“Oh, no reason.” He laughed as he restored Merlin’s hair back to his normal style.

  
  


* * *

  
  


“So, Merlin, what is your answer?”

The young warlock flung himself into the Prince's arms, kissing him hard. Arthur's soft sigh of relief as he was kissed back was all he needed to hear to know that he was making the right choice; and that all it cost him to arrive at this place was his dignity as a man, and his virginity.

**Author's Note:**

> Please visit my profile for more information on how to support me as a writer.


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